1. Field of the Invention
The present disclosure relates to a method of preparing a nuclear fuel pellet including a thermal conductive metal and a nuclear fuel pellet prepared thereby, and more particularly, to a method of preparing a nuclear fuel pellet having superior thermal conductivity by mixing thermal conductive metal powder.
2. Description of the Related Art
Nuclear power generation uses heat generated by the nuclear fission. Several tens to several hundred fuel pellets that are formed of a nuclear fuel material are put into a zirconium alloy cladding tube to seal each of both ends of the zirconium alloy cladding tube by welding, thereby manufacturing a fuel rod. Then, several fuel rods to several hundred fuel rods are bundled with each other to manufacture one assembly. The fuel rod assembly may be fed into a light water reactor or heavy water reactor. Here, heat generated from the nuclear fuel pellets is transferred into cooling water that flows around the fuel rods through the cladding tube via the nuclear fuel pellets.
A cylindrical pellet prepared by compacting and sintering a single or mixed material of oxides such as uranium (U), plutonium (Pu), or thorium (Th) may be used as a nuclear fuel that is used for nuclear power generation. Here, uranium dioxide (UO2) may be generally used as a material for forming the nuclear fuel pellet. In some cases, a nuclear fuel material in which at least one of other nuclear fuel materials such as oxides of Pu, Th, and Gd is added to uranium dioxide (UO2) may be used as the material for forming the nuclear fuel pellet. Particularly, the nuclear fuel pellet may be formed of (U,Pu)O2, (U,Th)O2, (U,Gd)O2, (U,Pu,Gd)O2, or (U,Th,Pu)O2.
According to a method of preparing an uranium oxide pellet that is a most widely-used nuclear fuel, a lubricant may be added to and mixed with uranium oxide powder that is used as a starting material to preliminarily compact the mixture at a pressure of about 1 ton/cm2, thereby preparing slug, and then, to pulverize the slug, thereby preparing granules. Thereafter, a lubricant may be added to and mixed with the prepared granules, and then, the mixture may be compressed/compacted and sintered under a hydrogen-containing gas atmosphere to prepare a fuel pellet. Here, the uranium oxide pellet that is prepared in the above-described process may generally have a cylindrical shape and a density of about 95% of a theoretical density (T.D.).
Also, (U,Pu)O2 or (U,Th)O2 pellets may be prepared through a method that is similar to the method of preparing the uranium oxide pellet after plutonium oxide (PuO2) or thorium oxide (ThO2) powder is mixed with uranium oxide powder. Also, (U,Gd)O2 pellets that are burnable absorber nuclear fuels may be prepared through a method that is similar to the method of preparing the uranium oxide pellet after gadolinium oxide powder is mixed with uranium oxide powder.
As described above, since the uranium dioxide (UO2) that is a representative nuclear fuel material has a high melting point and good compatibility with cooling water, the UO2 is widely used as the nuclear fuel material. However, the UO2 may have significantly low thermal conductivity of about 2 W/m·K to about 5 W/m·K in a workable temperature range. Here, due to the low thermal conductivity of nuclear fuel material in pellet, the fuel pellet may have a stiff temperature gradient. That is to say, the pellet may have a relatively high central temperature and a relatively low surface temperature. Here, a difference between the surface temperature and the central temperature of the pellet is inversely proportional to thermal conductivity. Thus, as the more the thermal conductivity decreases, the more the central temperature of the pellet increases. In the nuclear fuel rod that is normally burnt, the central temperature of the pellet may range from about 1,000° C. to about 1,500° C. Also, when a critical accident occurs, the UO2 may have a melting temperature of about 2,800° C. or more.
Also, since the nuclear fuel pellet has a high temperature and high thermal gradient, all reactions depending on the temperature may be accelerated to deteriorate material performance. Particularly, as a degree of burnup increases, the deterioration in material performance may increase.
Further, if the nuclear fuel pellet is in a high-temperature state, margin with respect to stability in various nuclear reactor accidents may reduce. For example, in case of a loss-of-coolant accident, when a temperature of the nuclear fuel just before the accident is high, the margin may be low. In case of a fuel-rod power ramping condition, since the pellet has low thermal conductivity, the central temperature of the pellet may be significantly higher than that in a normal operation condition. If the power is significantly limited to prevent the above-described problem from occurring, the fuel rod may not make a high output, the economic losses may occur.
To solve the problem in which the oxide nuclear fuel pellet has low thermal conductivity, Korean Patent Publication No. 10-2004-0047522 discloses a method of preparing a nuclear fuel containing a tungsten metal network, and more particularly, a method of preparing a nuclear fuel pellet including a tungsten metal network, in which a green pellet formed of nuclear fuel powder and tungsten oxide is heated under a reducing gas atmosphere to prepare a preliminary pellet, the preliminary pellet is heated under an oxidative gas atmosphere to form a tungsten oxide liquid network on the preliminary pellet, and the tungsten oxide liquid network is reduced to prepare the nuclear fuel pellet including the tungsten metal network.
However, according to the conventional preparing method as described above, two sintering processes may be performed under the oxidative gas and reducing gas atmosphere. This may be difficult from the existing commercial pellet fabrication process that is mainly performed under the reducing gas atmosphere. As a result, compatibility between the existing process and the above-described conventional method may occur. In addition, it may be difficult to change the gas atmosphere for each process during the processes.
Also, although the feature in which the liquid oxide is uniformly distributed along a grain boundary of the pellet like the related art is advantageous for the improvement in thermal conductivity, oxide volatilization in the liquid phase may occur.
Further, in the process of reducing the distributed liquid oxide, undesired pores may be formed in the pellet due to a change in volume and oxygen emission. Thus, poor fine structures may be formed in an outer portion of the pellet to deteriorate distribution uniformity of the metal material.
Thus, while the inventors study methods for effectively improving thermal conductivity of the nuclear fuel pellet, the inventors have developed a method in which, when a granule prepared from the oxide powder of nuclear fuel material are adequately adjusted in density and mixed with thermal conductive metal powder to prepare a pellet, the pellet is improved in thermal conductive while directly applying the existing pellet preparing process.